This invention relates to arc welding, cladding and brazing in general, and more particularly the present invention is directed to a process for controlling the temperature of continuously fed wires in such operations and an apparatus whereby the aforesaid process can be operationalized.
In conventional gas-metal arc welding (GMAW) processes, heating of the alloyed wire prior to deposition is accomplished by passing welding current through a certain wire length, commonly referred to as the wire stick-out. The power consumed in heating this wire is equal to the product of the square of the welding current and the resistance of the wire. In order to increase the wire deposition rate, the heat content of the wire is increased by increasing the electrical stick-out or the welding current or both.
Excessive wire stick-out leads to uncontrolled wire wandering and/or poor deposit quality. Therefore, welding current is the predominant variable that determines deposition rate and the mode of metal transfer through the arc. However, current intensification leads to more power dissipation in the arc. Since approximately 65 to 85% of the arc heat is conducted into the metal base, a higher current would increase arc penetration while increasing deposition rate and decreasing dilution. Although dilution can be reduced by employing higher welding current and slower speeds of travel, these means of control have practical limitations. Higher heat input per unit of length can generate excessive assembly distortion and metallurgical damage in both the deposit and the base metal, such as heat affected zone (HAZ) underbed cracking and hot cracking.
In the conventional hot wire gas-tungsten arc welding (HWGTAW) process, heating of the alloyed wire prior to deposition is accomplished by passing heating current through a certain length of wire stick-out. As in the GMAW process, higher wire heat content is adjusted by increasing wire stick-out or increasing heating current or both. This makes the HWGTAW process subject to similar difficulties experienced by the GMAW process.
My U.S. Pat. No. 4,447,703 entitled "Method and Apparatus for Arc Welding" which is assigned to the assignee of the present invention and incorporated herein by reference, teaches a technique whereby a consumable wire electrode is heated prior to insertion into the electrode stick-out region. This is accomplished by the circulation of current from a preheating power supply through a segment of the wire. Based on the teachings of the aforementioned patent, there is provided auxiliary in-line I.sup.2 R heating of the filler wire by adding an extra power supply and contact tip. This technique enables the substantial reduction of stick-out and enhanced positioning accuracy compared to conventional GMAW processes. For any given feed rate and length of wire (i.e., the distance, l, between the electric contact tips, the rigidity of the wire is inversely proportional to the current (I.sub.p) passing through it). The region enclosed by the "red" and "cherry-red" points corresponds to the softened condition of the wire. At this state, due to the lower strength and rigidity of the wire, frequent wire feed difficulties are most likely to develop. Obviously, this region must be avoided, wire preheating must be kept below the "red" state.
A simple and apparent way to prevent "wire-jamming" difficulties is to incorporate a monitor which continuously monitors the actual wire feed rate. Such monitoring enables one to "gate" the "safe" wire feed rate, so that the preheating conditions are maintained within safe limits. Thus, for example, if the wire feed rate was set to run at 300 inches per minute and preheat (I.sub.p) at 150 amperes, the wire feed monitor could be "gated" to turn off the welding operation if the wire feed rate suddenly dropped to, for example, 150 inches per minute. This turn-off feature would protect the system from "severe" wire "jam-ups". While this is a simple and apparent technique to prevent the wire jamming difficulties described above, there are certain disadvantages with "gating" the wire feed rate so that the preheating conditions are maintained within safe limits. Such disadvantages include: (1) depriving the preheating system of the flexibility to self-correct (i.e., self-adjust) the preheating conditions, to variations in the wire feed rate. In other words, due to the method's " rigidity", the system will unnecessarily be turned off more frequently than a self-adjusting method. Unless absolutely stopped, very erratic or drastically changed, most variations in wire feed should be accommodated without turning off the whole operation; and (2) this method does not control the thermal state of the wire even when it is fed within the safe gated range. If the wire feed rate varies from the one set, there is nothing to compensate and readjust the temperature of the wire at the new wire feed rate. Specifically, if the wire feed rate was lowered, but still within a safe range, and the preheating current (I.sub.p) did not change, the temperature of the wire will increase.
It is, therefore, an object of the present invention to provide a technique for algorithmically controlling temperatures of continuously fed wires, in order to eliminate substantially all the drawbacks associated with gating the wire feed rate.
It is a further object of this invention to provide a technique which will furnish a self-adjusting wire preheating system that maintains its desired temperature at any feed rate.